Fluidized beds of solid particles are employed in a variety of reactors, such as methanol to olefin (MTO) reactors, gas-phase polymerization reactors, and catalytic cracking reactors. Fluidized beds can also be found in reactors for treatment of pollutants via advanced oxidation processes. In reactors using a fluidized bed of solid particles, the solid particles typically serve as catalysts for the desired chemical reaction. For example, in MTO reactors the desired reaction is the conversion of methanol to an olefin such as ethylene or propylene. Methanol feedstock is passed through a fluidized bed of solid particles that catalyze this conversion.
One way to achieve greater control over the reaction in a reactor with a fluidized bed of solid particles is to monitor the state of the particles. Monitoring the condition of the solid particles within the reactor provides information for process adjustments to increase efficiency. For example, during operation of an MTO reactor, the solid catalyst particles in the fluidized bed will typically acquire some amount of amorphous carbon coating. This is sometimes referred to as ‘coking’ of the catalyst particles. Because the level of coking on the catalyst particles directly impacts the efficiency of the conversion reaction, MTO reactors typically include a regeneration system for reducing the level of coking on the catalyst particles. Monitoring the level of coking on the particles provides feedback for controlling this regeneration process.
Monitoring the state of the catalyst particles requires removal of a sample of the solid particles from the fluidized bed. Some current solid sampling methods for fluidized beds make use of lock hoppers with interlocked valves. A lock hopper refers to a volume within in an apparatus that can be isolated from the rest of the apparatus. Lock hopper designs are often employed to allow transfer of material between sections of an apparatus that operate at different pressures. In a system for collecting a sample of solid particles from a fluidized bed, the lock hopper is used to reduce the pressure of a sample from the higher pressure of the fluidized bed to a lower pressure (such as ambient) for testing of the sample. The interlocked valves prevent opening of a continuous path of valves that would allow escape of the pressurized particles from the fluidized bed.
An example of the lock hopper concept for sampling fluidized solids is shown in U.S. Pat. No. 3,614,230. According to the patent, a sample is drawn through a series of valves into a “deflaidization zone,” where the sample is isolated from the fluidized bed. This allows any fluidization gas (and excess pressure) to be removed from the sample by use of a filter. The amount of sample collected is determined by the amount of time the valves are left open After defluidization, the sample is transferred to another chamber for further processing.
One of the shortcomings of the aforementioned systems for sampling solid particles is that it is difficult to reproducibly collect a sample of a particular size. For example, in the apparatus used in U.S. Pat. No. 3,614,230, the size of the solid particle sample depends on the length of time the appropriate valves remain open as well as the flow rate of solid particles into the sampling system. As a result, the size of a solid particle sample collected in these conventional systems will be sensitive to small changes in process conditions.
What is needed is an effective method for collecting a small, reproducible sample of solid particles from a fluidized bed. The method should be self-limiting, so that precise timing is not needed to maintain a consistent size between successive solid particle samples. The method should also be easy to use, to allow for repeated sampling of a process if desired.